In an exhaust gas purification system for an internal combustion engine having an NOx storage reduction catalyst (which will be simply referred to as the “NOx catalyst” hereinafter) provided in an exhaust passage of the internal combustion engine, NOx contained in the exhaust gas is stored in the NOx catalyst when the air-fuel ratio of the exhaust gas is relatively high and the ambient atmosphere of the NOx catalyst is an oxidizing atmosphere. In this case, not only NOx but also SOx contained in the exhaust gas is stored in the NOx catalyst.
When oxide (i.e. NOx or SOx) stored in the NOx catalyst is to be reduced, it is necessary to decrease the air-fuel ratio of the exhaust gas flowing into the NOx catalyst (which will be hereinafter referred to as the “inflowing exhaust gas”) to a level that enables reduction of the oxide to be reduced.
Japanese Patent Application Laid-Open No. 2003-269155 discloses a technology in which when a reducing agent is added to the exhaust gas in order to decrease the air-fuel ratio of the inflowing exhaust gas when reducing NOx, the intake air quantity is decreased by controlling the degree of opening of an intake throttle valve and/or EGR valve. The air-fuel ratio of air-fuel mixture in an internal combustion engine can be decreased by decreasing the intake air quantity, whereby the air-fuel ratio of the exhaust gas can be decreased.